Set origin point to the center of circle?
If you select the circle and then press shift-s then a pie menu comes up. You can then press cursor to selection. This should put the 3d cursor in the center. If not, then your circle's origin is in the wrong spot. You can fix this bye, in object mode, press object with the circle selected. The object button is towards the top right. After you press this a dropdown will appear and you can hover over set origin, and then press origin to geometry. This will set the origin to the center of the object. Then you can repeat the step to set the 3d cursor to the selection and then the 3d cursor should be in the center of your circle. Hope this helps!